There are a variety of medical procedures which involve insertion of a catheter or slender tube into a body cavity or passage. The term "catheter" is used herein to describe all such devices, whether they be for probing a body passage, delivering or extracting a fluid, or for transporting an apparatus or instrument to a point within the body where a function is to be performed. "Catheter" thus means any invasive device, whether solid or hollow, which is intended for insertion in a body orifice or into an internal duct or passage through an external incision.
For example, catheterization may be needed in diagnosis and treatment of illnesses involving the brain, heart, lungs, bladder, etc. After the catheter is placed, it may be used to introduce or withdraw fluids from the cavity being probed, or it may be used as a carrier to support an instrument for measuring pressure or performing some other function in the cavity.
The catheter is inserted through an incision, or through the mouth or other body orifice, and is fed along an internal passage leading to the body area of interest. It is important to monitor the position of the catheter tip during insertion, because it is sometimes possible for the tip to be guided into the wrong passage at, for example, a Y-shaped junction of blood vessels or the like. Similarly, it is sometimes important to monitor the position of a catheter placed for long-term use to insure that the catheter has not moved within the body.
One catheter-locating technique which is well known in medicine involves coating, impregnating or otherwise positioning a radiopaque material on the catheter or at least the distal end portion of the catheter. The catheter position can then be determined by conventional X-ray procedures. This technique, however, requires expensive and bulky equipment operated by trained personnel, and involves a normally undesirable irradiation of the body with X-rays. The technique is also relatively slow, and can significantly extend the time required for catheter insertion which may require repeated monitoring of tip position as the catheter is moved toward the target position.
A more recent technique for catheter position monitoring is described in U.S. Pat. No. 3,847,157--Caillouette and Johnson disclosing a catheter which carries a magnetic material. Catheter position is monitored with a movable external magnetic-field sensor which indicates proximity to the catheter magnetic material. This arrangement appears satisfactory in many applications, but may not provide a clear position indication of a deeply implanted catheter, and is also susceptible to interference from stray or ambient magnetic fields.
The catheter position monitor of this invention uses a catheter with a coupling or field sensing device at the catheter distal tip, the device having connecting wires extending back through the catheter tube to an amplifying and detecting circuit external to the body. An external transmitter is positioned to generate an energy field which penetrates the body and is sensed by the device at the catheter tip. The relatively weak signal received from the device is externally processed to provide amplitude and phase information which enables determination of the position and orientation of the catheter tip.
The invention permits use of a strong and deeply penetrating energy field because the field generator is external and not subject to the size and electrical limitations which characterize implantable devices. Connection of the sensor to external equipment enables amplification of weak signals to a level convenient for further processing and display. The hazards, inconvenience and expense of X-ray position monitoring are avoided, and catheter position is quickly and easily determined both during insertion and after placement of the catheter.